She had to have a minor procedure done and had to stay overnight. When we picked her up they told us "she" was a "He". I asked if I was in trouble with the pet misgender police? Good comeback from the front desk told me "as long as you pay the bill, we don't care what pronouns you use!"
Same thing happened to us - feral cat that we and two of our rural neighbors fed and provided sleeping space in outbuildings. The neighbors always referred to seeing ‘her’, and she was friendly with our very large male tabby. She moved into our house before the winter - only took her 8 years to make that decision. We called her Lucy the whole time, but after a vet visit a month ago, it’s now Mr. Lucy.